Secret radio communication system



Vmmh 8, 1949.

C. E. ATKINS -SECRET RADIO COMMUNICATION SYSTEM.

2 S'heets-Shet l Filed Feb. 18, 1943 ATTORNEYS Patented Mar. 8, 1949 SECRET RADIO COMMUNICATION SYSTEM Carl E. Atkins, Evanston, Ill., assignor to Tung- Sol Lamp Works, Inc., Newark, N. J., a corporation of Delaware Application February 18, 1943, Serial No. 476,273

Claims.

My present invention relates to secret point-topoint communication systems of the general type disclosed in my copending applications Ser. No. 443,898, led May 21, 1942, and Ser. No. 468,345, filed December 9, 1942. The system of the. present invention differs from those of my earlier filed applications in that amplitude rather than frequency scrambling is employed. My present system comprises an end-to-end oscillator of which the oscillatory feed-back paths between the two oscillators constitute the respective signaling channels. As in the systems of my earlier filed applications, signaling energy introduced at one station is used to scramble the energy introduced at the other station; the signal energy introduced at each station making a complete circuit of the system before elimination therefrom. The present invention also includes novel means for preventing frequency drift of the oscillators employed for frequency conversion and also-novel arrangements for augmenting camouflageof the exchanged intelligence. The new system, as hereafter described, is particularly adapted for telegraphic communication.

Of the accompanying drawings,

Fig. 1 is a diagrammatic layout of the apparatus of either of two stations between'which secret communication is to be had;

Fig. 2 is a circuit diagram representing suitable apparatus for introducing into the system signal energy varying both in frequency and in amplitude; and

Fig. 3 is a circuit diagram illustrating an alternative arrangement for introducing signal energy into the system.

In Fig. 1 the equipment at either station is shown as including a receiver 2 adapted to receive energy from a receiving antenna 4 and a transmitting apparatus 6 connected to the receiver 2 through intermediate apparatus hereinafter described and arranged to deliver energy to a transmitting antenna 8.

The intermediate apparatus includes a demodulating unit III shown as a multi-element tube having the cathode and one lcontrol grid connected across the output terminals of the receiver 2 and a second control grid connected to the output of a balanced modulator 63 which in turn -receives energy from the output of buer and delay filter I2. Delay lter I2 connects to an audio frequency unit I4 which includes a source of audio frequency signal energy and means for generating special telegraph signals of intermittent and capricious wave-form. This unit will be more fully described in connection with Figs. 2 and 3. A limiter I6 and a detector unit I8 are connected in parallel to the output circuit of demodulating unit III.

A suitable filter circuit I1 is preferably interposed between unit I0 and limiter I6 and detector I8. Limiter I6 serves to control the overall amplitude of the end-to-end oscillation. A buffer amplifier 20 is connected to receive energy from limiter I6 and to deliver such amplified energy to one grid of a complex modulating tube 22.

An audio-frequency responsive unit 24, which may be earphones, or other reproducing device, is connected to the output circuit of detecting unit I8. Also connected to the output circuit of unit I8 is a filter 26, the output voltages of which are impressed upon a grid of the modulating tube 22. A third grid of the tube 22 is connected directly with unit I4.

Modulator tube 22 delivers energy through a suitable filter 28 to transmitting unit 6.

The apparatus so far described can be considered as that at either of two communicating stations, that at the other station being substantially a mirror image of that illustrated in Fig. l. All the respective stages in the oscillatory chain represented by the two stations and the communication channels therebetween are linear amplifiers and must be amplitude responsive. The reference amplitude is determined by the limiter IB which sets the level for the radio frequency carrier upon which the scrambled signals appear as amplitude modulation.

In the following description of the operation of the apparatus so far described the subscript "a will be used to identify apparatus at one station, say station A, andthe subscript b will be similarly used to identify apparatus at the other station, say station B.

In operation, a signal originating in unit Ida is applied to duplex modulator 22a so that an amplified modulated signal is radiated from transmitting antenna 8a. This signal proceeds through the apparatus at station B Where it may or may not receive additional amplitude modulation, depending upon operation of unit I 4b, as will hereinafter become apparent, and returns as modulation at a different communications frequency to the receiver 2a. During this time, the amplitude modulation introduced by unit Illa has been passing through delay network I2a and is applied to balanced modulator 63a Where it combines with carrier energy applied to the cooperating input terminals of 63a from buffer Zlla. The resulting output comprising sideband energy is applied to the grid of demodulator Illa at precisely the same time the modulated signal returns from station B. This arrangement serves to cancel the modulation appearing on the signal as a result of original modulation in unit Ill'a. The energy appearing at amplitude detector |80. will thus be devoid of modulation unless such has been introduced into the system as unit Mb of station B. If such is the case this signal from station B will be rectified by unit 10a and will pass on to the audio frequency reproducing equipment 24a where it will be available as a signal conveying intelligence from station B. The radio frequency energy from demodulator 10a is also passed to limiter 16a which serves to wipe out all amplitude modulation so that nothing but a radio frequency carrier is applied to buffer stage 20a and through this stage to complex modulator tube 22a. The modulation energy from station B, after passing through detector I 8a is fed to phase-shift network 20a and through this device to the additional grid of duplex modulator 22a. This technique thus serves to reintroduce the amplitude modulation from unit Mb of station B that had been removed from the carrier by the action of limiter 10a. If now, unit Ilia continues to generate a signal, the energy thereof is applied to the third grid of unit 22a and consequently the radio frequency energy passing through transmitter a and radiated by antenna 0a to station B will contain an amplitude scrambled signal in which there are those components originally generated in unit trib of station B ,together with the new components introduced by unit Ma of station A. Duplex modulator 22 must be so operated that crossmodulation does not occur between the audiofrequency energies from lter 26 and audiofrequency unit I4. These energies could of course be mixed in any suitable way and applied to a common grid in modulator 22.

It is desirable to transmit on a different radio frequency from that used for reception and hence the radio frequency energy in receiver 2 and transmitter 0 will differ in frequency while a common frequency will serve as the carrier joining the receiver and transmitter. In a complex oscillatory system such as the end-to-end oscillator system above described, trouble may be encountered because of frequency flutter or motorboating. Frequency flutter occurs when one of the oscillators, the frequency of which is used for heterodyning, has an operating frequency deviating slightly from the precise period required for conversion to the correct frequency. Such deviation causes a cumulative frequency shift unless means are provided for counteracting the effect thereof. In my copending application -Serial No. 468,345, filed December 9, 1942, I have disclosed an arrangement involving the operation of oscillators at opposite sides of the transmission frequency together with inversion of the replica signal, which arrangement effectively prevents oscillator drift, and could be employed in the present system. I prefer, however, to employ in the present system the arrangement now to be described.

In Figl an oscillator 30 supplies a beating frequency to the receiver 2, which it will be understood contains suitable means for mixing the energy from antenna 4 with that from the oscillator 3 to deliver energy of the resultant frequency to the demodulating tube I0. Oscillator 30 also supplies beating frequency to a mixer 32 the output of which is delivered to transmitter as the beating frequency for conversion of the intermediate frequency to the transmitting frequency. A self-excited oscillator 34 supplies the other beating frequency to mixer 32. The frequency of oscillator 34 is that of the desired difference in the frequency of the incoming and outgoing energy of the station and this selfexcited oscillator is locked or stabilized by the feeding of a portion of the energy from a mixer 36 receiving energy from antenna il and from antenna 8 through suitable amplitude limiters or lters 38 and L30 respectively.

The operation of the above described apparatus will be best understood if numerical values of the frequency of the various units are assumed. Thus, if the frequency transmitted from station B to station A is 2100 kc., and that transmitted from station A to station B is 1700 kc., then the diiference frequency and that of self-excited oscillator 34 at each station will be 400 kc. If the intermediate frequency of the energy passing through the apparatus between receiver 2 and transmitter 5 at each station is 100 kc., then oscillator 30a should have a frequency of 2000 kc., or 2200 kc. If this frequency is 2000 kc., then the output from mixer 32a, will be 1600 kc., which is the correct value for heterodyning with the kc., intermediate frequency for transmission at 1700 kc. Should the frequency of oscillator 30a now deviate from the desired value of 2000 kc., say to 2001 kc. then the intermediate frequency becomes 99 kc., but this in transmitter 6a will now heterodyne with 1601 kc., from mixer 32a instead of 1600 kc., and hence the transmitted frequency will be the correct value of 1700 kc., in spite of the deviation of oscillator 30a. Due to the provision of mixer 36a, receiving energy of 2100 kc., from antenna llav and 1700 kc., from antenna 8a deviation of the frequency of oscillator 34a from 400 kc., is prevented.

The apparatus at station B is of course the same except for the numerical values of the operating frequencies. For the values heretofore given, oscillator 30h will be a 1600 kc. oscillator, 34h will be a 400 kc. oscillator, mixer 32h will pass the sum frequencies, that is 2000 kc. to transmitter 0b.

Ihe above described arrangement for correcting for frequency drift, while particularly adapted for use in the present system, could be advantageously employed in any system involving compound heterodyning, whether the networks connecting the system are, as here, antenna systems and open space, or whether they arephysical connections.

As indicated diagrammatically in Fig. 1, the unit I4 of each station includes a key l5 for the sending of telegraphic symbols. The actual elements comprising eachunit Ill will depend upon the type of keying to be applied to the carrier. For example, when unit I4 is keyed a D. C. potential may be applied to duplex modulator 22 and after a suitable time delay and phase-reversal in unit I 2, to demodulator I0 as well. Thev oscillatory amplitude would then increase or decrease, depending upon the polarity of. this D. C. pulse. These pulses would be detected as dots and dashes at the other station. They would likewise be so interpreted by an interloper unless the other station simultaneously keyed the common carrier so as to mask these pulsations in amplitude. If the same magnitude of change were always used by the apparatus it would then be possible for the interloper to analyze the signal and assign the point of origin to any and all pulses. If, however, these pulses differ in amplitude in an irregular-manner, the source thereof -would be hidden.

Instead of D. C. pulses, the system can be keyed with a modulating tone and in Figs. 2 and 3 alter; native arrangements of unit I4 for augmenting camouage when keying is effected by use of a modulating tone are disclosed.

In Fig. 2 the keying tone at each point has its frequency changed at random so that no single frequency characterizes a given station, and further, the percentage of modulation is changed at random so that unauthorized deciphering is impossible. In Fig. 2 the apparatus includes an audiofrequency oscillator 42 arranged to be controlled by the key I5. The frequency determining portion of oscillator 42 is shunted by a reactance tube 44, which in turn is controlled from a source 45 of capricious voltage. An electronic volume control is represented by the multi-element tube 4B, of which one grid is connected to receive energy from a source 48 of capricious voltage feeding through a low pass filter 50. The output of tube 48 appears across a load impedance 52 and is applied through suitable coupling means, here shown as condensers 54 and 56 to units 22 and I2 of Fig. 1. With the above described apparatus, unless key I 5 is depressed, the

end-to-end R. F. carrier is not modulat-ed by energy originating in the corresponding unit I4. However, when the key is depressed, a tone originating in oscillator 42 is applied in the form of amplitude modulation to the carrier. During the interval of the telegraphic symbol the tone frequency will vary in an irregular manner because of the action of the capricious voltage 45, the reactance tube 44 and the oscillator 42. Thus the pitch of the modulating tone is varied at random. Also, as the transconductance of the tube 46 is varied at random by the voltage fed through the lter 5I) from the source 48, the amplitude of the voltage across the impedance 52 will vary at random. Thus the arrangement of Fig. 2 insures keying with a modulating tone which varies capriciously not only in frequency but also in amplitude.

Although for simplicity two separate sources 45v and 48 of capricious voltage have been indicated in the drawing, obviously one such source could Vcontrol both tubes 44 and 46.

Instead of using capriciously varying modulating tones at the two stations, successful camouflage of the signals may be obtained by the use of precisely the same tone at each station, provided that the two tones not only are precisely the sam-e in frequency but also in phase. If these tones were out of phase or their phase relationship varied from moment to moment it would be possible to tell when two signals were present instead of only one. The availability of knowledge of this kind would greatly impair the secrecy of the system. When the modulating tone does the same thing to the R. F. carrier regardless of where we introduce it and when further the degree of modulation applied when the respective telegraph keys are depressed is changed at random it then becomes impossible to decipher the radiated intelligence. In Fig. 3 is illustrated apparatus suitable for use when the same modulating tone is used at each station.

' In Fig. 3 a frequency divider 58 is arranged to receive energy from self-excited oscillator 34 (see Fig. l) the frequency of which is locked, as heretofore described, at the value of the difference between the transmitted frequencies. Thus the frequency of the energy supplied to the frequency divider 58 at each station Will be identical and its phase angle should be constant, although Cir not necessarily identical, at ythe two stations. Frequency divider 58 may be any type known to the art, as, for example, one employing cascaded counter circuits. Key I5, in this case, serves to apply output from divider 58 to a terminal of an electronic volume control unit 60, which may be a multi-element tube such as tube 46 of Fig. 2. A source of capricious voltage 52 actuates the electron volume control 68 so that its output will vary in an unpredictable manner. An adjustable phase-shifting network 64 is connected in the output circuit of unit 60 so that correction can be made for phase differences in the locking energy of oscillator 34a and 34h. After passage through the network 64, the output from unit 6I] is applied to modulating tube 22 and delay filter I2. Using the above described arrangement it would be impossible for an interloper to determine whether both keys or but one key was depressed, inasmuch as the common keying tone would be changing its amplitude irregularly.

It will be apparent from the above description that the invention provides a. novel and practical station-to-station secret communication system using amplitude scrambling for interchange of signals. The invention includes also a method of and means for preventing oscillator drift. Alternative arrangements for introducing the signal energy at each station have been illustrated and described. Various features of the systems of my co-pending applications Ser. No. 443,898, led May 21, 1942, and Ser. No. 468,345, led December 9, 1942 could be employed in the present system as will be apparent to those skilled in the art. For example, the means for producing a delayed replica of the signal energy, herein indicated diagrammatically as delay lter I2, could be the particular means disclosed in application Ser. No. 443,898. Also, tape mechanisms, such as indicated in Ser. No. 468,345 could be included in unit I4 to provide continual scrambling when but one-way communication is desired.

If the transmitting frequencies between the stations are not substantially differenty shielding means between the antennae 4 and 8 of each station should be employed or, preferably, the concurrency system of my co-pending application Ser. No. 449,256, filed July 1, 1942, now Patent No. 2,426,581 granted Sept. 2, 1947, which permits concurrent reception and transmission on the same frequency, should be employed.

I claim:

1. A secret station-to-station communication system comprising in combination receiving and transmitting means at each station, and apparatus at each station intermediate the receiving and transmitting means, said apparatus including demodulating and modulating devices, signal introducing means having a connection with the modulating device to impress audiofrequency amplitude variations upon the energy delivered to the transmitting means and having a connection to said demodulating device to suppress corresponding amplitude variations in the energy delivered from the receiving means to the demodulating device, said last mentioned connection including a delay filter, an amplitude limiter between said devices and a by-pass about said limiter including a detector for reintroducing at the modulating device the amplitude variations in the energy delivered by said demodulating device and suppressed by said limiter.

2.` A secret station-to-'station communication system comprising in combination receiving and aeeasoa transmitting means. at4 each. station, and: apparatus at: each stationy intermediate the receiving and transmitting means, saidv apparatus including demodulating. and modulating devices, signal introducing means having; a connection with the modulating device to impress audio-frequency amplitude variations upon the energy` delivered tothe transmitting means and having a. connection to said demodulating device. to suppress corresponding amplitude variations; in. the energy delivered from the receiving means: to. the demodulating device, said last mentioned connection including a delay iilter, an amplitude' limiterbetween said devices and a by-pass abouti said limiter including a detector for reintroducing at the modulating device. the amplitude variations: in: thel energyY delivered by said demodulating device and suppressed by said limiter, and including at each. station an oscillator associated with the receiving means for providing a beating. fre;- quency for conversion of the received energy to energy of an intermediate frequency, a self-excited oscillator at each station locked toy a fre,- quency corresponding to the. difference frequency of the received and transmitted energy, and means for mixing the energy from said oscillators to provide energy of a beating frequency for conversion of the energy of intermediate frequency to the frequency for transmission.

3. A secret station-to-station communication system comprising energy transmitting means and energy receiving means' at each station, a source ofv signal energy at each station,. controllable means at each station for unpredictably amplitude modulating the energy transmitted from the station with the signal energy from said source,` means at each stationfor scrambling; the modulating signal energy of that station with: the modulating signal energy originating at the: other station and radiated therefrom to the first station, and means at each station for suppressing modulation in the received energy correspending to that previously introduced at` the same station, whereby modulation energy introduced at each station is eliminated from the sys.- tem upon return to the station at which introduced.

4. A secret station-to-station communication system. comprising energy transmitting means and energy receiving means at each station,- a.

source of signal energy at each station,y controllable means at each station for unpredictably amplitude modulating the energy transmitted from that station with the signal energy from said source, means at each station for scrambling the modulating signal energy of that station with the modulating signal energy originating at the other station and radiated therefrom to the iirst station, means at each station for suppressing modulation in the received energy corresponding to that previously introduced at the Same station whereby modulation energy'introduced. at. each station is eliminated from the system upon return to the station at which introduced, said controllable means at each station including an electronic volume control adapted When operat.- ing to deliver amplitude modulating signal energy into the system, a source of capricious volt-- age connected to said volume control for capriciously varying the amplitude of the energy delivered thereby, and key-controlledv means for rendering said. volume control operative.

5. A secret station-to-station communication system comprising in combination energy receiving and'. transmitting. means at each station,.

8 aV source of. signal energy at each station, means capriciously varying both the. frequency and amplitude of the. signal energy from. said source, means for modulating the energy transmitted by said transmitting means. with the; varying signal energy from the source at the same station and with. the. varying signal energy originating at the other station and radiated therefrom to the rst station asA modulation of the transmitted energy, and means: at each station for suppressing. modulationI in the received` energy corresponding to that previously introduced at. the same.l station, whereby modulation energy introduced aty eachy station is eliminated from the; system. upon return tothe station at which introduced.

6; In. a. secret station-to-station communication system'v of the type. wherein energy of one trequency is. transmitted from one station for reception at the. other station and energy at a different. frequency is transmitted from. the other station for reception at the iirstv station, the improvement comprising, at each station an oscillator locked to the difference frequency of the radiatedl energy from both stations, a. frequency divider receiving energy from said oscillator, an electronic volume control: arranged' when energized to supply modulating signal energy to the system, a source of capricious. voltage; connected tofsaidvolume control for varying the amplitude of the energy delivered thereby, and. key means for connecting said divider with said. volume. con.- trol whereby the modulating tone ati both sta,- tions has the same frequency.

'7'. In a4 secret station-to-stati'on communication system of thel type whereinenergy-of one fre;- quency is transmitted from one. station. for reception at the other station and energy ata. dfferent frequency is transmitted from the' other station for reception at the rst station, the im- 40 prOVement COmDISDg at eachstation` an oscillator locked to the diierence. frequency of the radiated energy from both stations, a frequency divider receiving energy from said oscillatory. anv electronic volume control. arranged when, eneri gized to supply modulating signal energy to the system, a source of capricious voltage connected to said volume control for varying the amplitudeA of the energy delivered thereby, key means, for connecting said divider with said volume control whereby the modulating tone at both stations; has the same frequency, and means; at. each, station for adjusting the phase of the. energy de.;- liveredl by said' volumecontrol toV provide, parity of. phase of the modulating tones;

8. In a secret station-to-station communication` system the combination comprising a transmitter and a receiver at` each station, means` at each station for amplitude modulating the en.- ergy to be radiated by said transmitten, aA de.- modulating device at each station so connected with said means as to suppressamplitude: modulation inthe energy received by the receiver and. previously introduced into the system by said means at that station, whereby modulation energy introduced at each stationis: eliminated from the system. upon return to the. station aty which introduced, adetector connected toY said demodulating, device for detecting amplitude modulation introduced at the other station. and a. modulating device forl reintroducing into the system the. amplitude introduced at the otherv station together with continuing amplitude mod;- ulation originating at said rst mentioned means. of the first station.

9;. In; a secret station-to-statioir communication system the'combination comprising a transmitter and a receiver at each station, means at each station for amplitude modulating the energy to be radiated by said transmitter, a demodulating device at each station so connected with said means as to suppress amplitude modulation in the energy received by the receiver and previously introduced into the system by said means at that station whereby modulation energy introduced at each station is eliminated from the system upon return to the station at which introduced, a detector connected to said demodulating device for detecting amplitude modulation introduced at the other station, and a modulating device for reintroducing into the system the amplitude modulation introduced at the other station together with continuing amplitude modulation originating at said first mentioned means of the first station, said means for amplitude modulating the energy to be radiated varying the modulating tone capriciously in frequency and in amplitude and wherein a key is provided for rendering said means operative.

10. In a secret station-to-station communication system the combination comprising a transmitter and a receiver at each station, means at each station for amplitude modulating the energy to be radiated by said transmitter, a demodulating device at each station so connected with said means as to suppress amplitude modulation in the energy received by the receiver and previously introduced into the system by said means at that station whereby modulation energy introduced at each station is eliminated from the system upon return to the station at which introduced, a detector connected to said demodulating device for detecting amplitude modulation introduced at the other station, and a modulating device for reintroducing into the system the amplitude modulation introduced at the other station together with continuing amplitude modulation originating at said first mentioned means of the first station, and a self-excited oscillator at each station locked to the difference frequency of the energies radiated by said stations for insuring equality of frequency of the modulating tones introduced at the two stations by said means for amplitude modulating the energy to be radiated.

11. In a secret station-to-station communication system the combination comprising a transmitter and a receiver at each station, means at each station for amplitude modulating the energy to be radiated by said transmitter, a demodulating device at each station so connected with said means as to suppress amplitude modulation in the energy received by the receiver and previously introduced into the system by said means at that station whereby modulation 'energy introduced at each station is eliminated from the system upon return to the station at which introduced, a detector connected to said demodulating device for detecting amplitude modulation introduced at the other station, a modulating device for reintroducing into the system the amplitude modulation introduced at the other station together with continuing amplitude modulation originating at said first mentioned means of the first station, and means at each station for converting the frequency of the energy received at that station to energy of an intermediate frequency and for converting energy of said intermediate frequency to energy of a higher frequency for radiation by the transmitter, said means comprising an oscillator, a self-excited oscillator, a mixer receiving energy from both of said oscillators and a mixer receiving energy of the frequency of that received by the receiver and energy of the frequency of q that radiated by the transmitter, said last mentioned mixer delivering energy to said self-excited oscillator to lock the frequency thereof to the difference frequency of the received and transmitted energy, said lrstmentioned mixer delivering energy of beating frequency for conversion of the energy of intermediate frequency to the transmitted frequency and the first mentioned oscillator delivering energy of beating frequency for conversion of the frequency of the received energy to the intermediate frequency whereby compensation is made for frequencydrift of the first mentioned oscillator.

12. The method of secret signalling between two stations which comprises simultaneously amplitude modulating a carrier wave at each station with audio frequency signal energy introduced at that station and with audio frequency signal energy introduced at the other station and transmitted thereby as amplitude modulation to the first station, unscrambling the received energy at each station by suppressing the amplitude modulation in the carrier corresponding to the signal previously introduced at that station and transmitted therefrom to the other station and back to the first station, whereby modulation energy introduced at each station is eliminated from the system upon return to the station at which introduced, and thereafter detecting at each station the signal introduced at the other station.

13. The method of secret signalling between two stations which comprises simultaneously amplitude modulating 'a carrier wave at each 4o station with audio frequency signal energy introduced at that station and with audio frequency signal energy introduced at the other station and transmitted thereby as amplitude modulation to the rst station, unscrambling the received energy at each station by suppressing the amplitude modulation in the carrier corresponding to the signal previously introduced at that station and transmitted therefrom to the other station and back to the first station, whereby modulation energy introduced at each station is eliminated from the system upon return to the station at which introduced, and thereafter detecting at each station the signal introduced at the other station, including capriciously varying the modulating tone of the introduced signal energy and capriciously varying the amplitude of such modulating energy.

14. The method of secret signalling between two stations which comprises simultaneously amplitude modulating a carrier wave at each station with audio frequency signal energy introduced at that station and with audio frequency signal energy introduced at the other station and transmitted thereby as amplitude i modulation to the first station, unscrambling the received energy at each station by suppressing the amplitude modulation in the carrier corresponding to the signal previously introduced at that station and transmitted therefrom to the other station and back to the rst station, whereby modulation energy introduced at each station is eliminated from the system upon return to the station at which introduced, and thereafter detecting at each station the signal introduced at the other station, including utilizing the energy received and the energy transmitted .at eachstation to control .the frequency of the modulating tone of the signal energy at that station to thereby make the .frequency of the modulating tones at both .stations identical, while Ecapricioursly varying the amplitude of the modulating energy during introduction into the system.

,15. The method of preventing .frequency drift in a system :according to claim 3 which 4comprises mixing energy of the frequency received. 10 Number with 'energy .of the :frequency transmitted at each station to lock a self-'excited oscillator at the differ-.ence 1freqgziencies of :such energies, mixing the energy ,from such locked oscillator with energy from another oscillator and utilizing ,such mixed 15 energy for one frequency leonversion of the system and the energy of the last mentioned oscil- 12 lator for v.another trequency 'conversion of the system.

CARL E. ATKINS.

.REFERENCES :CITED The following references are of record in the file 'of this patent:

UNITED 'STATES PATENTS Name Date 2,-034gf'f75 Rooseustein Mar. 24, 1936 2,`l.-58,6.62 Kotowski May 16, 11939 2,201,650 Purington June 1i, 1940 FORETGN PATENTS Number Country :Date

288,714 Great Britain MW. Api'. 12, 1938 Certificate of Correction Patent No. 2,463,504. March 8, 1949.

CARL E. ATKINS It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 8, for as unit read at unit; column 7, line 36, claim 3, for the station read that station; column 8, line 71, claim 8, after amplitude insert modulation; column 9, line 38, claim 10, after the Word and comma station, strike out and and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealedthis 30th day of August, A. D. 1949.

[SEAL] THOMAS F. MURPHY,

Assistant Uommz'ssz'oner of Patents.

Certificate of Correction Patent No. 2,463,504. March 8, 1949.

CARL E. ATKINS It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 8, for as unit read alt um't; Column 7, line 36, claim 3, for the station read that station; column 8, line 71, claim 8, after amplitude insert 'modulation; column 9, line 38, claim 10, after the Word and comme stationf strike out U andi? and that the said Letters Patent should be read With these corrections therein that the same may conform to the record of the oase in the Patent Office.

Signed and sealed this 30th day of August, A. D. 1949.

THOMAS F. MURPHY,

Assistant Uommz'ssz'oner of Patents. 

